1. Technical Field
The present disclosure relates to an integrated torsional-microbalance device in MEMS technology and to a corresponding fabrication process.
2. Description of the Related Art
A microbalance, typically obtained with MEMS technology, is a mechanical structure driven by an electrical signal that causes an oscillatory movement thereof, preferably at the resonance frequency of the mechanical structure. FIG. 1 shows a block diagram of a control system 1 of a known type used for actuating a microbalance. The control system 1 comprises a first transducer 2, which receives on an input of its own a driving signal Vin generated by an appropriate driving electronics 4 and, on the basis of the driving signal Vin received, generates a force F on a microbalance 10, such as to cause oscillation preferably at a resonance frequency f0, specific for the mechanical structure of the microbalance 10. External events, such as, for example, a change in the mass of the microbalance 10 due to deposition of material M on the microbalance 10, cause a shift Δf0 of the frequency of oscillation from the resonance frequency f0. A second transducer 3 converts the oscillations of the microbalance 10 into an electrical signal, generating at output an output signal Vo that varies in a way depending upon the frequency of oscillation of the microbalance 10. The output signal Vo is then supplied to a processing electronics 5 for subsequent processing steps.
FIGS. 2 and 3 show, respectively in top plan view and in perspective view, a possible embodiment of the microbalance 10 and of part of the control system 1 (the driving electronics 4 for generating the driving signal Vin and the processing electronics 5 for processing the output signal Vo are not shown). The control system 1 of this type, together with the microbalance 10, is described, for example, in “A CMOS-compatible bulk technology for the fabrication of magnetically actuated microbalances for chemical sensing”, A. Nannini, D. Paci, F. Pieri, P. Toscano, Sensors and Actuators B, vol. 118, pp. 343-348, 2006, and in “A CMOS-compatible, magnetically actuated resonator for mass sensing applications”, D. Paci, F. Pieri, P. Toscano, A. Nannini, Sensors and Actuators B, vol. 129, pp. 10-17, 2008.
With joint reference to FIGS. 2 and 3, the microbalance 10 comprises a mobile body 11, including a main portion 17 preferably of a square or rectangular shape, supported by a first arm 12 and by a second arm 13. The first and second arms 12, 13 possess a respective end that is fixed with respect to the main portion 17, whilst the other end is fixed with respect to a substrate 14. The first and the second arms 12, 13 are moreover aligned along an axis of symmetry 15 passing through the centroid of the mobile body 11.
The mobile body 11 possesses at least one opening 16, preferably of a square or rectangular shape and having its own centroid set on the axis of symmetry 15. The opening 16 has the function, during the steps of fabrication of the microbalance 10 (as will be described better in what follows), of enabling removal of the underlying layer/layers of material (for example, the substrate 14) and providing a suspended structure, supported by the first and second arms 12, 13.
The mobile body 11 further comprises an actuation winding 20 and a detection winding 21, integrated in the mobile body 11 on different metal levels (for example, the detection winding 21 is formed on a first metal level, and the actuation winding 20 on a second metal level, or vice versa). The actuation winding 20 comprises a first connection portion 20′ and an actuation loop 20″, both of which are made of conductive material and are electrically connected to one another, whilst the detection winding 21 comprises a second connection portion 21′ and a plurality of concentric detection loops 21″, all of which are made of conductive material and are electrically connected to one another.
The first connection portion 20′ provides an input port 22 of the actuation winding 20, and the second connection portion 21′ provides an output port 23 of the detection winding 21.
The actuation loop 20″ traverses the main portion 17 along its perimeter, parallel to the sides of the main portion 17. Likewise, also the detection loops 21″ traverse the main portion 17 along its perimeter and can be partially superimposed on the actuation loop 20″ but not in direct electrical contact therewith.
A pair of magnets 25 (or, alternatively, a single permanent magnet) are set in the proximity of the microbalance 10, on opposite sides thereof, so as to generate a magnetic field B having field lines with direction perpendicular to the axis of symmetry 15 passing through the first and second arms 12, 13. The magnets 25 can be, for example, permanent magnets made of neodymium-iron-boron (NdFeB), having dimensions smaller than 1 mm3, which are able to generate a magnetic field of approximately 0.1 T.
If on the input port 22 of the actuation winding 20 an input voltage Vin is applied to generate a current Iin through the actuation winding 20, on the sides of the microbalance 10 orthogonal to the field lines of the magnetic field B, as is known, the Lorentz force F is exerted, given byF=l·Iin·iB  (1)where l is the length of the portion of the actuation winding 20 that lies on the side on which the Lorentz force F is exerted, and i is a unit vector with a direction parallel to the portion of the actuation winding 20 on which the Lorentz force F is exerted and the same sense as the current Iin.
The Lorentz force F is, instead, zero on the sides of the microbalance 10 parallel to the field lines of the magnetic field B.
There is hence generated on the microbalance 10 a twisting moment τ, which induces a rotation of the mobile body 11 about the axis of symmetry 15
                    τ        =                              ∑            i                    ⁢                                    F              i                        ⋀                                          b                _                            i                                                          (        2        )            where Fi is the force that acts on the i-th loop of the actuation winding 20, and bi is the arm of the moment that acts on the i-th loop of the actuation winding 20.
From Eqs. (1) and (2) we find that the magnitude τm of the twisting moment τ is given byτm=2blIB=AinIB,  (3)where Ain is the area subtended by the actuation winding 20, and B is the magnitude of the magnetic field B.
In the specific example of FIG. 2, the actuation winding 20 comprises a single loop, and consequently the twisting moment τ is only due to the Lorentz force F that acts on opposite sides of the single loop of the actuation winding 20.
From Eq. (3) it may be assumed that the mobile body 11 of the microbalance 10 oscillates with a frequency of oscillation proportional to the frequency of the current Iin. During the oscillations of the mobile body 11, the first and second arms 12, 13 have the function of torsional springs. If the frequency of the current Iin corresponds to the resonance frequency of the microbalance 10, the amplitude of the oscillations is a maximum.
The oscillations of the mobile body 11 are detected, in the presence of the magnetic field B, by the detection winding 21. As is known, according to the Faraday-Lance law, the oscillation of the mobile body 11 in the presence of the magnetic field B causes an increase in the flux of the field through the loops of the detection winding 21, generating an electromotive force that opposes the variation of flux across the detection winding 21 (output port 23). If the mobile body 11 rotates rigidly about the axis of symmetry 15, the output voltage Vout on the output port 23 is given by
                              V          out                =                              -                                          ∂                                  Φ                  B                                                            ∂                t                                              =                                    A              out                        ⁢            B            ⁢                                                            ∂                  sin                                ⁢                                                                  ⁢                θ                                            ∂                t                                                                        (        4        )            where ΦB is the flux of external magnetic field B, Aout is the area subtended by the detection winding 21, and θ is the angle of rotation of the mobile body 11. If the angle θ of rotation of the mobile body 11 is sufficiently small, Eq. (4) can be approximated as follows:
                              V          out                ≈                              A            out                    ⁢          B          ⁢                                          ⁢                                    ∂              θ                                      ∂              t                                                          (        5        )            
A possible application of the control system 1 regards the detection of complex organic molecules such as DNA or proteins. In this application, the use of the control system 1 provides a series of advantages as compared to commonly used optical-reading methods, based, for example, upon fluorescence techniques. In the first place, since no cumbersome optical detectors are required, it is possible to provide a totally integrated control system 1. In the second place, reading is extremely simple and affords an improved sensitivity, enabling detection of the presence of particular molecules and supply of a measurement of the weight, and hence of the amount, of said molecules. Finally, it is not necessary to pretreat the DNA or the proteins with an optical label.
However, since the mass of the molecules or of the compounds to be detected is normally extremely small, the sensitivity of MEMS microbalances of the type described generally proves insufficient for detecting concentrations of molecules below a certain minimum-amount threshold.